The use of wireless communication systems is growing with users now numbering well into the millions. One of the most popular wireless communications systems is the cellular telephone system in which a mobile station (or handset) communicates with a base station. Cellular telephones allow a user to talk over the telephone without having to remain in a fixed location. This allows users to, for example, move freely about the community while talking on the phone.
Antenna arrays may be used in communications systems that transmit and/or receive radio frequency signals. Antenna arrays typically include a number of antennas that are spatially separated and may be employed in a number of different wireless applications including radio communications systems, cellular systems, television broadcasting, point to point systems, paging systems, medical applications or the like.
The use of antenna arrays in such systems provides antenna performance improvements over the use of a single element antenna. Such antenna performance improvements for received signals may include improved signal to noise ratio and interference rejection. Antenna performance improvements for transmitted signals may include improved gain, reduced interference, increased network, and lower transmit power requirements. Antenna arrays may be used for signal reception only, for signal transmission or for both signal reception and transmission.
A typical application of antenna array systems is in a wireless communication system. Examples include a cellular communication system and a wireless local loop system. Such wireless communications systems typically include one or more communications stations, generally called base stations, each communicating with subscriber units, also called remote terminals, mobile stations, and handsets. In cellular systems, the handset is typically mobile, while in wireless local loop systems; the handset is typically in a fixed location.
The antenna array is typically at the base station, but may also be employed at a user terminal. Communication from the mobile station to the base station is typically called the uplink and communication from the base station to the mobile station is typically called the downlink. In time division duplex (TDD) systems, uplink and downlink communications with a particular remote terminal occur at the same frequency, but at different time slots. In frequency division duplex (FDD) systems, uplink and downlink communications with a particular remote terminal occur at different frequencies and may or may not occur at the same time.
Since different frequencies are used in the uplink and downlink communication channels in FDD, the behavior of the signals in the uplink and downlink communication channels between the base station and remote terminal are different. The differences are more apparent as the channels have more multipath components. In a multipath environment, scattering and/or reflections caused by, for example, buildings, hills, trees, and the like result in amplitude and phase changes and/or multipath components in the communication signals. Multipath components behave differently for different frequencies and a communication signal arriving at an antenna array changes with the frequency. Accordingly, the optimal signal combining function at an adaptive antenna array based wireless communications system is itself a function of scatterers in the environment.
Another factor that impacts wireless communication system performance is the presence of multiple remote terminals utilizing the same frequency or channel at the same time in the environment. As the number of separate sources or remote terminals utilizing the same frequency increases, the amount of interference in the communications channel also increases.
Adaptive antenna array signal processing can dramatically reduce the amount and effects of communications channel interference. If the adaptive antenna system can account for all environmental scattering effects, the extent to which interference can be reduced is greatly improved. We propose a means by which an FDD system wherein frequency dependent scattering might otherwise limit the performance of interference suppression of an adaptive antenna array is operated in a combined FDD TDD mode. Operating in this combined mode allows the array signal processing system to overcome the limitations of FDD operation.